This invention relates to an electrochemical doping procedure for the selective modification of the room temperature electrical conductivity properties of conjugated polymers and to the application of such procedure in the design of novel high energy density secondary batteries.
For use in a wide variety of electrical and electronic device applications, it is highly desirable to have available electrically conducting materials having a preselected room temperature electrical conductivity varying over a broad range extending from slightly conducting to highly conducting. Furthermore, particularly for use in semiconductor device applications, requiring one or more p-n junctions, such materials should advantageously be available with both p-type and n-type electrical conductivities.
It has recently been found that semiconducting acetylene polymers, such as polyacetylene, can be chemically doped in a controlled manner with electron acceptor and/or electron donor dopants to produce a whole family of p-type and n-type electrically conducting doped acetylene polymers whose room temperature electrical conductivity may be preselected over the entire range characteristic of semiconductor behavior and into the range characteristic of metallic behavior. Such doping procedures and the resulting doped acetylene polymers are described and claimed in the commonly assigned U.S. patent application Ser. No. 902,667, of Alan J. Heegar, Alan G. MacDiarmid, Chwan K. Chiang, and Hideki Shirakawa, filed on May 4, 1978 now U.S. Pat. No. 4,222,903; and in the commonly assigned U.S. patent application Ser. No. 902,666, of Alan J. Heeger, Alan G. MacDiarmid, Chwan K. Chiang, and Shek-Chung Gau, also filed on May 4, 1978 now U.S. Pat. No. 4,204,216; both of said applications being incorporated herein by reference. As described in said Heeger, et al. applications, a p-type material is obtained with electron acceptor dopants, and an n-type material is obtained with electron donor dopants. The resulting room temperature electrical conductivity of the doped acetylene polymer increases with increasing degree of doping up to a certain point at which the maximum conductivity is obtained for any given dopant, such maximum conductivity generally being obtained at a degree of doping not greater than about 0.30 mol of dopant per --CH-- unit of the polyacetylene.
The doping procedures described in said Heeger, et al. applications involve merely contacting the acetylene polymer with the dopant, which may be either in the vapor phase or in solution, whereby uptake of the dopant into the acetylene polymer occurs by chemical reaction to a degree proportional with both the dopant concentration and the contacting period, such concentration and contacting period being coordinated and controlled so that the corresponding degree of doping will be such as to provide the resulting doped acetylene polymer with the preselected room temperature electrical conductivity. While such doping procedures are generally effective for achieving the desired result, they are subject to certain limitations. First of all, it is rather difficult with these procedures to obtain a reliably precise control of the degree and uniformity of doping so as to ensure commercial scale production of a doped polymer product with a consistent and uniform room temperature electrical conductivity. Secondly, the fact that these procedures require a doping material which is chemically reactive with the acetylene polymer is a limiting factor in the selection of doping materials which are economically attractive. Thirdly, these procedures require the use of two totally different doping materials and, consequently, separate doping systems for carrying out p-type and n-type doping. Hence, the development of more efficient and economical doping procedures would greatly enhance the commercial attractiveness of doped acetylene polymers as substitutes for the more conventional electrically conductive materials.
Doped acetylene polymers constitute one class of recently developed molecular solids exhibiting relatively high levels of electrical conductivity. Several of these other molecular solids have previously been investigated as electrode materials in attempts at improved battery design. For example, the Moser U.S. Pat. No. 3,660,163, issued May 2, 1972, and Schneider, et al., Proc. Int. Power Sources Conf., 651-659 (1974), describe the use of a charge transfer complex of iodine and poly-2-vinylpyridine with excess iodine as a cathode material in a solid-state lithium-iodine primary battery employing lithium iodide as a solid electrolyte. While this prior art battery is characterized by a relatively high energy density, it suffers from several drawbacks. First of all, it is a primary battery, i.e., it is not capable of being recharged. Secondly, in order to avoid the problems which might be caused by undesired flow of the viscous charge transfer complex and undesired diffusion of the excess free iodine from the cathode mixture, it is necessary for the battery to be constructed with various internal protective coatings and containment materials, which increase the weight and size of the battery and reduce its energy density. Furthermore, the output current which the battery is able to deliver, both in relation to its electrode area and in relation to its weight, is rather low.
A recent article by Yoshimura, appearing in Molecular Metals, edited by William E. Hatfield, NATO Conference Series, Seris VI: Materials Science, pp. 471-489 (1978), at pages 474-476, refers to the above-described prior art solid-state lithium-iodine primary battery constructed with poly(vinylpyridine)iodine charge transfer complex cathode material, and broadly speculates that a number of the molecular metals, including doped polyacetylene, might possibly find similar utility as cathode materials in battery design. However, no further details are provided in regard to the possible construction or mode of operation of such hypothetical batteries. Furthermore, the possibility of doped acetylene polymers, or other doped conjugated polymers, being employed as one or both of the electrode materials in a secondary battery construction, i.e., in batteries which are capable of being charged and discharged over many cycles, is not even hinted at in this article.